Production of super phosphoric acid



United States Patent O 3,134,644 PRODUCTION OF SUPER PHGSPHORKC ACID Raymond J. Shatiery, Robert H. Smith, and Harvey F. Greening, aii of Lawrence, Kane, assignors to FMC Corporation, a corporation of Delaware Filed July 27, 1961, Ser. No. 127,323 4 Claims. (Cl. 23-165) This invention relates to a method for producing super phosphoric acid from elemental phosphorus.

Super phosphoric acid, i.e., about 104 to 106% H PO has been in increasing demand for use in fertilizer formulations. This is due to the higher assay liquid fertilizers which can be obtained by using super phosphoric acid, thereby placing them on a competitive basis with highassay solid fertilizers. Additionally, freight costs for shipping super phosphoric acid are reduced on a P basis even beyond the expected savings for weight alone since 105% super acid occupies 58% of the volume of 75% phosphoric acid.

In order to supply this increased demand, super phosphoric acid production has been attempted on commercial equipment designed to produce normal, i.e., about 75 phosphoric acid. One such commercial installation for producing normal phosphoric acid is described in US. Patent No. 2,708,620, issued to Henry S. Winnicki on May 17, 1955. In this process, phosphorus is burned with air to form P 0 within an unlined, metal-wall, tower. The P 0 is absorbed by liquid films of phosphoric acid which run down the inside of the tower and accumulate in a sump at the base of the column. The accumulated acid in the sump is pumped to a heat exchanger for cooling and returned to the upper sections of the tower to maintain the films of phosphoric acid flowing down the interior walls of the tower. The utilization of the existing equipment employed in this process for producing super phosphoric acid has given rise to most serious problems, particularly when an increased rate of conversion of phosphorus to phosphoric acid is attempted.

At the outset, super phosphoric acid is quite viscous and cannot be pumped normally at the temperatures utilized in producing 75 phosphoric acid. The heat transfer of super phosphoric acid at these temperatures is very low because the film of acid nearest a heat exchange wall, i.e., the heat resistant film, is quite thick. Thus, operation at these temperatures can be carried out only with great difficulty and with extremely small production. Materially increasing the temperature of the super phosphoric acid, i.e., about 180 C. makes it more readily flowable but also increases the rate of corrosion beyond allowable limits, i.e., about 7 mils per year (m.p.y.), resulting in rapid corrosion of the heat exchange equipment. As a result high production of super phosphoric acid without exceeding the permissible corrosion rate has not been readily achieved.

It is an object of this invention to produce super phosphoric acid in a wetted-wall unlined metal-walled tower in high yields without corroding the process equipment.

These and other objects will be apparent from the present disclosure.

It has now been determined that super phosphoric acid, i.e., about 104 to 106% H PO can be produced by burning elemental phosphorus in a wetted-wall, unlined, metalwalled tower, without exceeding 7 m.p.y. corrosion, by operating the process so that the temperature of the super phosphoric acid in the tower is between 130 to 140 C.

Operation at these temperatures more than doubles the super phosphoric acid production capacity of these units compared with operations at 120 C. This surprising increase in production capacity is due to a change in flowability of the super phosphoric acid at about 130 C. At temperatures below about 130 C. the super phosphoric 'ice acid will flow only when subjected to high pressures. As a result, if super phosphoric acid at a temperature below 130 C., i.e., about C., is passed from the base of the wetted-wall tower into heat transfer tubes for cooling, high pressures are required to pass the acid through. In order to obtain normal operating flow rates, pressures would have .to be used which are above the pressure limits of the heat transfer means. Since this cannot be done without rupturing the heat transfer tubes, the rated pressure limits of the heat exchanger restrict the pressure which can be applied to drive the super phosphoric acid through the heat exchanger. This in turn fixes the maximum rate of flow of super phosphoric acid throughout the system. This rate is greatly below the desired rate.

In contrast when the super phosphoric acid reaches a temperature of about C. it becomes readily flowable. As a result, it can be pumped from the sump of the wettedwall tower through the heat exchangers and cooled at rapid flow rates without employing pump pressures which exceed the pressure limitations of the heat exchange equipment. At a temperature of 130 C. the super phosphoric acid has a corrosion rate which is well below 5 m.p.y. The corrosion climbs with increase temperature until it reaches 7 m.p.y. at 140 C. Beyond this amount, corrosion becomes excessive and must be avoided. The preferred operating temperature for super phosphoric acid is about C., since this gives excellent flowability, with minimum corrosion of the heat exchange equipment. Temperatures within the range of 130 C. and C. can be utilized with good results since operation with phosphoric acid at these temperatures satisfies the two critical requirements of the instant process, namely, good flowability and lack of excessive corrosion.

The invention will now be described more particularly with reference to the attached drawing. In the drawing a combustion furnace for the burning of elemental phosphorus to phosphoric pentoxide is illustrated at 1 and consists preferably of a vertical stainless steel tower (AlSI type 316 stainless steel) having a phosphorus burner 2 adjacent to the top thereof which receives phosphorus from line 3 and air for atomizing molten phosphorus from line 4. Sufficient air for complete combustion of the phosphorus enters the top of the tower through suitable openings 5. A phosphorus burner of this general type is described in greater detail in Chemical Engineering, volume 55, No. 10, page 105, October 1948.

The dome 9 of the tower 1 is cooled by water sprayed from the ring 25 connected to the cooling water line 13 and the inside of the tower dome is cooled by water from the ring 26 sprayed against the underside of the dome. A dam 27 around the outer edge of the dome 9 maintains a pool of water at the outer edge of the dome at all times.

Surrounding the top open end of the tower 1 below the dome thereof is a launder 6 forming a reservoir into which recirculating phosphoric acid may be pumped from the line 7. Suitable weirs, indicated in dotted lines inside the launder 6, insure uniform distribution of the acid along the walls of the tower 1. Additional recirculating acid may be sprayed into the tower from the line 7a by spray pump 7b through spaced spray nozzles 8 located in one or more positions along the walls of the tower 1. The nozzles 8 may be in two or three tiers and are uniformly spaced around the tower. The phosphoric acid overflows the weirs, indicated in dotted lines around the top of the tower 1 inside the launder 6, and flows down and covers the interior walls of the tower 1 from top to bottom thereof. Additional spray inlets 8 permit additional phosphoric acid to be introduced along the interior walls of the tower as needed to keep the walls uniformly wetted and provide sufiicient acid to absorb the phosphorus pentoxide.

If required, water may be sprayed into the tower through some of the inlets 8 and additional water sprays may be located in the dome 9 of the tower. The phosphoric acid flowing along the walls of the tower absorbs phosphorus pentoxide, and water which is vaporized from the water sprayed into the tower assists in hydration and absorption of the phosphorus pentoxide vapors into the phosphoric acid solution flowing along the walls of the tower.

A level of liquid is maintained approximately at the dotted line position indicated as 10 in the bottom of the tower so as to maintain a suitable pool or reservoir of phosphoric acid for recirculation through the tower. Such a pool protects the bottom of the tower 1 from con tact with the combustion gases and increases the absorption of phosphorus pentoxide therein. It will be understood, however, that the reservoir of phosphoric acid for recirculation through the tower can be maintained in tanks outside the tower instead of in a pool inside the base of the tower.

The flow of acid through the tower is so regulated that super phosphoric acid is drawn from the bottom of tower 1 through the line 11 preferably at a temperature of approximately 135 C. and at a concentration of approximately 105% H PO.;. The withdrawn acid is pumped by weir pump (circulating pump) 11b to the heat exchangers 12 from which it emerges at a temperature of approximately 95 C. A portion of the withdrawn acid is passed to storage through line 11a and the remainder is pumped back through the line 7 to launder 6.

The outside of the tower 1 is cooled by a film of cooling water 24 which is sprayed around the top of the tower 1 from the ring 24a connected to the cooling water line 13. The cooling water from the line 13 is preferably at a temperature of approximately 25 C. and after flowing down the walls of the tower is discharged to the sewer or to storage reservoirs at a temperature of approximately 60 C.

The uncondensed gases and unabsorbed phosphoric pentoxide are withdrawn from a level near to but above the base of the tower 1 through the outlet 14, which is located above the liquid level 10 and preferably slopes upwardly as indicated, and passes through the line 15 to a Venturi scrubber 16, or any other means for the removal of mist from a gas stream such as a packed tower or an electrostatic precipitator. In the Venturi scrubber 16 the gases are contacted with phosphoric acid pumped from pump 7b through line 13 and into scrubber 16 to absorb any remaining phosphorus pentoxide and scrub out the remaining phosphoric acid. In the Venturi scrubber 16 the remainder of the phosphorus pentoxide is precipitated and recovered as a liquid flowing through the line 17a. The uncondensed gases after passing through the Venturi scrubber 16 are at a temperature of approximately 115 C. The remaining moisture consists largely of phosphoric acid droplets, and can be separated in the moisture separator 19 and returned to the tower through line 20. The unabsorbed gases then pass through line 21 and are discharged to the atmosphere through the stack 22. A slightly positive pressure is maintained in tower 1 by a fan not shown to withdraw the steam and gases from the outlet 14. By simultaneously flowing water down the outside walls of the tower 1 and flowing phosphoric acid along the inside walls of the tower 1, it is possible to maintain the wall of the tower below the temperature at which stainless steel is corroded by super phosphoric acid. When operating the process to produce 105% phosphoric acid, it is desirable to withdraw the phosphoric acid at the bottom of the tower 1 at approximately 135 C., and to return phosphoric acid to the top of the tower at approximately 95 C., and to discharge the uncondensed gases and unabsorbed phosphorus pentoxide from the outlet 14 at 4 approximately 140 C., although other suitable operating temperatures may be maintained.

The following example for the production of super phosphoric acid, according to the process described above, is presented as illustrative of the present invention and is not intended as limitative of the invention.

EXAMPLE 1 Super phosphoric acid was produced in an acid furnace of the type described in FIGURE 1. The unit employed a Venturi scrubber to remove unabsorbed phosphorus pentoxide or phosphoric acid mist from the gas stream. A Ducon mist separator was employed to remove any moisture containing phosphoric acid from the stack gases. The conditions of operating, including the flow rates, temperatures and pressures employed in carrying out the present example, are given in Table 1.

Example 1 demonstrates the preferred embodiment of the present invention. If operations were attempted using the same equipment but at a temperature of C., the pressure required to drive the super phosphoric acid through the system, and particularly the heat exchangers, would be well above the rated pressure limits of the existing heat exchangers. As a result operations at 120 C. is impossible with the equipment set up under the same conditions as Example 1. Operations at 120 C. can be carried out only if the heat exchangers are hooked up in a parallel arrangement, rather than in the series arrangement of Example 1. When this is done the pressure within the heat exchangers decreases to acceptable limits but the productivity of super acid drops well below half the value obtained in Example 1.

Table 1 Furnace operating conditions: Results P burning rate (lbs/hr.) 5580. Percent H PO 104.7. Number of sprays 2 banks. Heat exchangers 2 (in series).

Flow rates:

Dome water spray (g.p.m.) 3.8. Weir acid flow (g.p.m.) 930. Spray acid flow (g.p.m.) 530. Heat exchange, water flow (g.p.m.) 1200. Venturi recirculation (g.p.m.) 71. Product acid tap-off (g.p.m.) 17.5. Atomizing air, s.c.f.m. 190. Secondary air, s.c.f.m 8530.

Temperatures, C.:

Furnace acid 135.9. Acid after heat exchanger 96.7. Water before heat exchanger 22.8. Water after heat exchanger 62.3. Shell water inlet 22.8. Shell water discharge 52.6.

Pressure, p.s.i.g.:

Weir pump (circulating pump) discharge 131. Spray pump discharge 105. Spray pressure 80. Corrosion rate (m.p.y.):

Heat exchanger (at hottest point) 4.6.

Pursuant to the requirements of the patent statutes, the principle of this invention has been explained and exemplified in a manner so that it can be readily practiced by those skilled in the art, such exemplification including what is considered to represent the best embodiment of the invention. However, it should be clearly understood that, within the scope of the appended claims, the invention may be practiced by those skilled in the art, and having the benefit of this disclosure, otherwise than as specifically described and exemplified herein.

What is claimed is:

1. The method of producing super phosphoric acid in concentrations of about 104 to 106% H PO from elemental phosphorus, which comprises burning phosphorus in a thin-walled, unlined, stainless steel tower to produce phosphorus pentoxide in vapor phase inside said tower, introducing an aqueous phosphoric acid solution of about 104 to 106% H PO into said tower and flowing said acid in a thin film along the inner walls of said tower covering the walls of said tower from top to bottom thereof, introducing water into said tower, hydrating and absorbing said phosphorus pentoxide vapors into the flowing phosphoric acid film on the walls of said tower while flowing water along the outside of said tower to provide additional heat transfer from the acid flowing along the inner walls of said tower, and maintaining the walls at a temperature of about 130 to 140 C., removing the uncondensed gases from the tower at approximately 140 C., removing acid from the tower at between 130 to 140 C., cooling the acid outside the tower to from 80 to 100 C., recirculating the acid along the inner walls of said tower, and recovering as product phosphoric acid from said tower in concentrations of 104 to 106% H PO 2. The method of producing super phosphoric acid in concentrations of about 104 to 106% H PO from elemental phosphorus, which comprises burning phosphorus in a thin-wa1led, unlined, stainless steel tower to produce phosphorus pentoxide in vapor phase inside said tower, introducing an aqueous phosphoric acid solution of about 104 to 106% H PO into said tower and flowing said acid in a thin film along the inner walls of said tower covering the walls of said tower from top to bottom thereof, introducing water into said tower, hydrating and absorbing said phosphorus pentoxide vapors into the flowing phosphoric acid film on the walls of said tower while flowing water along the outside of said tower to provide additional heat transfer from the acid flowing along the inner walls of said tower, and maintaining the walls at a temperature of about to 140 C., collecting phosphoric acid solution in a pool in the bottom of the tower, removing the acid from the pool, cooling said acid outside the tower and recirculating said acid along the inner walls of said tower, removing unabsorbed phosphorus pentoxide and the gases from the tower near to but above the pool in the base of the tower at a temperature between 130 to 140 C., recovering additional phosphoric acid from said removed gases, and recovering as product phosphoric acid from said pool in the base of said tower in concentrations of from 104 to 106% H PO 3. Process of claim 1 in which the temperature of the phosphoric acid is C.

4. Process of claim 1 in which the super phosphoric acid is recovered at a concentration of 105% References Cited in the file of this patent UNITED STATES PATENTS 2,247,373 Hartford et a1 July 1, 1941 2,303,318 Baskervill Dec. 1, 1942 2,708,620 Winnicki May 17, 1955 2,999,010 Striplin et a1. Sept. 5, 1961 3,015,540 Striplin et a1. Jan. 2, 1962 

1. THE METHOD OF PRODUCING SUPER PHOSPHORIC ACID IN CONCENTRATION OF ABOUT 104 TO 106% H3PO4 FROM ELEMENTAL PHOSPHORUS, WHICH COMPRISES BURNING PHOSPHORUS IN A THIN-WALLED, UNLINED, STAINLESS STEEL TOWER TO PRODUCE PHOSPHORUS PENTOXIDE IN VAPOR PHASE INSIDE SAID TOWER, INTRODUCING AN AQUEOUS PHOSPHORIC ACID SOLUTION OF ABOUT 104 TO 106% H3PO4 INTO SAID TOWER AND FLOWING SAID ACID IN A THIN FILM ALONG THE INNER WALLS OF SAID TOWER COVERING THE WALLS OF SAID TOWER FROM TOP TO BOTTOM THEREOF, INTRODUCING WATER INTO SAID TOWER, HYDRATING AND ABSORBING SAID PHOSPHORUS PENTOXIDE VAPORS INTO THE FLOWING PHOSPHORIC ACID FILM ON THE WALLS OF SAID TOWER WHILE FLOWING WATER ALONG THE OUTSIDE OF SAID TOWER TO PROVIDE ADDITIONAL HEAT TRANSFER FROM THE ACID FLOWING ALONG THE INER WALLS OF SAID TOWER, AND MAINTAINING THE WALLS AT A TEMPERATURE OF ABOUT 130* TO 140*C., REMOVING THE UNCONDENSED GASES FROM THE TOWER AT APPROXIMATELY 140*C., REMOVING ACID FROM THE TOWER AT BETWEEN 130* TO 140*C., COOLING THE ACID OUTSIDE THE TOWER TO FROM 80* TO 100*C., RECIRCULATING THE ACID ALONG THE INNER WALLS OF SAID TOWER, AND RECOVERING AS PRODUCT PHOSPHORIC ACID FROM SAID TOWER IN CONCENTRATIONS OF 104 TO 106% H3PO4. 